Electrophysiologic Study vs. Electrocardiographic Monitoring (ESVEM) is a multicenter, investigator-initiated, clinical trial which began enrollment on 10/1/85. The investigators hypothesize that either electrophysiologic study (EPS) or Holter monitoring (HM) is more accurate in identifying antiarrhythmic drug efficacy in patients with ventricular tachyarrhythmias (VTA). Subjects with inducible (VTA) and greater than or equal to 10 ventricular ectopic beats per hour are randomized at 12 enrolling centers to undergo serial drug efficacy testing by one or the other method. They may receive up to six antiarrhythmics in random order until one is predicted to be effective by suppression of inducible VTA in the EPS limb or suppression of ectopy in the HM limb. Estimation of the median time to arrhythmia recurrence in subjects with an efficacy prediction is the primary analysis by which we will compare the two testing methods. On 2/15/90, 407 subjects had been randomized and a drug efficacy prediction had been achieved in 236. The goal is to enter 290 patients into long-term follow-up on drugs predicted effective by one of the testing methods. This goal should be reached by 5/31/91, and the trial will conclude one year later. A three year period of funding is needed for completion of the trial, data analysis and reporting. Twelve additional objectives include: A study of the concordance between HM and EPS; a comparison of the relative cost and the cost-effectiveness of the two approaches; an evaluation of the role of HM in monitoring long-term drug efficacy; an examination of heart period variability as a predictor of malignant arrhythmias; delineation of the influence of left ventricular dysfunction upon antiarrhythmic drug efficacy; and use of body surface potential mapping to identify arrhythmia vulnerability. Considerable controversy persists over whether patients with VTA should receive antiarrhythmic therapy based upon EPS or HM. This study will help clarify most issues in that controversy. It will provide new information concerning risk assessment in patients with VTA which may help define the subgroups at greatest risk, as well as those more likely to benefit from therapy. This project will develop refinements in the application of EPS and HM and in the interpretation of their results.